MT-CO1 belongs to cytochrome c oxidase subunit I protein family. It is often used in DNA barcoding, because its mutation rate is often fast enough to distinguish closely related species. In most if not all seed plants, however, the rate of evolution of cox1 is very slow.

The enzyme complex consists of 3-4 subunits (prokaryotes) up to 13 polypeptides (mammals) of which only the catalytic subunit (equivalent to mammalian subunit I (CO I)) is found in all heme-copper respiratory oxidases. The presence of a bimetallic centre (formed by a high-spin heme and copper B) as well as a low-spin heme, both ligated to six conserved histidine residues near the outer side of four transmembrane spans within CO I is common to all family members.